Apricity OS is an Arch-based Linux distribution with a simple, beautiful, and consistent UI:

According to the developers, "Apricity OS idles at around 500 megabytes of memory usage, as opposed to other popular operating systems that idle at up to several gigabytes. This also helps bring boot times down to just a few seconds." I found it fast and responsive running in a VM with just 2GB of RAM. Love the little coffee cup icon in the menu bar which functions much like OS X's caffeinate, though it has the added benefit of disabling the screensaver as well.